1. Field of the Invention
The present invention relates to an image processing apparatus that can read out an original that is bound together such as a book or magazine, and more particularly, to an image processing apparatus that can read out an image of an original with fidelity.
2. Description of the Related Art
An image reading apparatus called a book scanner is commercialized. This apparatus has an image sensing system disposed above an original platen with an open space of several ten centimeters in height between the original platen and the image sensing system. To read an original in a bound format such as a book or magazine (referred to as "book original"), the user sets the book original in a spread manner facing upwards on the platen. A lamp providing light to the original is turned on in response to the start key being depressed, whereby a scanning operation on the original plane (the side face to be read out) is initiated. An image read out by scanning is appropriately output to an external device. If the external device is a printer, the read out image is printed out. It is to be noted that the book original includes documents filed together.
A book original generally has an image such as text also on the back side of the spread page. In other words, an image is recorded on both sides of a sheet. When the sheet is very thin, an "opposite side transmittance" phenomenon where an image of the back side is discernible through the surface side occurs. This opposite side transmittance degrades the quality of the read out image significantly.
In the field of copying machines, the technique of reducing the effect of such opposite side transmittance by means of image processing is proposed. In the copy operation of a sheet original on which an image is printed on either side of the sheet (duplex original), a density A read out from the front surface side is compared with a density B read out from the back side for each pixel in the original image. A pixel of density B greater than density A is regarded as an opposite side transmittance pixel, and density A of the opposite side transmittance pixel is replaced with the background density (for example 0).
In reading an original of just one sheet, only the issue of opposite side transmittance is to be taken into consideration. In reading a book original which has a plurality of sheets superimposed in a spread manner, there is a possibility that, not only an image of the back side of the spread out page, but also an image of an underlying page is perceptible from the surface of the spread out page. The same can be said in the case where there are a plurality of leaves of documents bound by a stapler and the like.
In the above-described conventional method of image processing, data correction is carried out for each pixel in the read out original on the basis of only one of two choices, i.e. maintain density A, or replace the pixel of interest with background density. The degree of opposite side transmittance depending on the material of the sheet is not reflected in data correction. Therefore, there was a problem that an image read out with fidelity of the original image cannot be obtained when there is a half tone image in both the front and back sides of the sheet with the half tone images of each side overlying each other.